The substrate of the present invention is of the type formed by a sintered stack of sheets of dielectric material, at least some of which have conductive patterns, and having conductive portions (tracks or mere rims) emerging at its surface for subsequent connection to the terminals of the component. At least one internal layer of the substrate is also provided with conductive tracks for providing interconnections between layers (between internal layers or between an internal layer and an upper layer) and with emerging portions following a pre-established pattern.
Alumina is known to have advantageous properties in constituting substrates for electronic components when compared with conventional ceramics: excellent dielectric properties (in particular a small increase in the tangent of the loss angle as a function of frequency); high thermal conductivity (thus avoiding the appearance of hot points and facilitating heat sinking for power components or for very highly integrated components); good mechanical properties; and a good surface state enabling very fine conductors to be applied directly thereto, e.g. by photochemical etching.
The excellent high frequency behavior makes it possible to use such substrates at microwave frequencies or with very high speed logic circuits (having a switching period of about 100 ps) in which conventional ceramics cannot be used.
However, these various advantageous properties are only fully obtained for "ultra-pure" type alumina compositions, i.e. containing at least 99% Al.sub.2 O.sub.3 and preferably at least 99.9% Al.sub.2 O.sub.3. Such composition requires high temperature firing, at about 1600.degree. C. to 1800.degree. C., and the purer the alumina, the higher the temperature required.
The compositions currently used for making alumina substrates generally comprise 80% to 96% Al.sub.2 O.sub.3. The resulting products have about 80% of the mechanical properties and 60% to 70% of the thermal properties of ultra-pure alumina. In contrast, the tangent of the loss angle increases or varies abnormally as a function of frequency, which prevents such alumina being used in microwave applications. Further, the firing temperature remains about 1600.degree. C.
For example, U.S. Pat. No. 4,340,635 describes a composite structure substrate formed on a base of 96.6% alumina covered with a surface layer of 99.4% alumina (the invention of this patent is concerned with obtaining an ultra-pure alumina surface state on alumina which is only 96.6% pure). The firing temperature is 1575.degree. C. to 1675.degree. C. and is preferably 1640.degree. C.
When it is desired to provide an interconnection substrate (i.e. a substrate which includes ab initio buried metallization rather than metallization added to the alumina after firing), this range of temperatures requires the use of refractory metals for making the buried interconnection tracks, e.g. molybdenum or tungsten, or an alloy of these metals (currently used non-oxidizable metals and alloys or have melting temperatures in the range 960.degree. C. (silver) to 1550.degree. C. (palladium), which are lower than the temperatures required for firing, which are about 1600.degree. C.).
A first drawback of this technique results from the oxidizable character of these metals: the substrate must thus be fired under a reducing atmosphere in a controlled atmosphere oven (e.g. under a hydrogen atmosphere). Further, a substrate obtained in this way cannot withstand subsequent firing under an oxidizing atmosphere (e.g. ambient air), e.g. when adding additional surface conducting tracks at a later stage.
A second drawback comes from the formation during co-sintering of a mixed diffusion oxide at the metal-alumina interface. Although this feature improves the bonding of conductive tracks on the surface of the substrate, it also has the drawback of forming buried conductors having poor geometry because of the diffusion zones. It is thus not possible to make very fine buried tracks for high density interconnections, nor to eliminate the risk of breaks occurring during co-sintering (or short circuits occurring between conductors which are too close together).
A second technique for making interconnection substrates is also known, in which the firing takes place under an oxidizing atmosphere (e.g. ambient air) and at lower temperature, of about 900.degree. C. to 1270.degree. C. To do this, the composition is modified by adding silica to the alumina together with oxides of alkali metals or alkali-earth metals (magnesia, lime, soda, potash) so as to come closer to a porcelain or to a ceramic glass.
It then becomes possible to use non-oxidizable and non-meltable metals at the maximum temperatures obtained during firing, e.g. palladium, gold, silver or alloys of these metals. European patent specification No. 0 045 877 describes an example of a substrate of this type.
However, the properties of the resulting substrates are not comparable to the properties of substrates made of pure or ultra-pure alumina. The mobility of the alkali additives increases the electrical conductivity of the dielectric, which also becomes temperature sensitive. A high concentration of silica loses the exceptional thermal and mechanical qualities of alumina and comes closer to those of glass (poor thermal conductivity, fragility). In any event, microwave applications are still not possible.
A third technique derived from the second consists in a first step of firing a base of pure or ultra-pure alumina followed by silk screening conductors on its surface, and then covering the conductors with a layer of ceramic glass, and finally firing the assembly at low temperature (850.degree. C. to 900.degree. C.). The steps of silk screening, covering, and firing are then repeated as many times as required to obtain a plurality of buried interconnection levels.
However, the possibilities of this technique are limited to a small number of interconnection levels by virture of the cummulative diffusion of metal during successive applications of firing. Further, since the intermediate layers are not made of alumina, the electrical and thermal performance of the resulting assembly is limited.
The invention aims at eliminating these drawbacks and at escaping from the comprising solutions of the prior art, by proposing an interconnection substrate of alumina having high performance in spite of being made by means of simplified technology. The invention also aims at providing a method of manufacturing such a substrate.
More precisely, one of the objects of the invention is to obtain an interconnection substrate which combines the properties which, up to now, have been exclusive to ultra-pure alumina, namely: the excellent dielectric properties required for microwave applications; high thermal conductivity enabling effective heat sinking for dissipating high levels of heat generation on a small surface area; directly obtaining a good surface state; low porosity; mechanical robustness; the possibility of providing very thin dielectric layers; the possibility of providing many such layers and at various thicknesses; and uniform shrinkage during firing.
Another aim of the invention is to make it possible to provide interconnection substrates under an oxidizing atmosphere with a maximum firing temperature which is reduced to a value of about 1400.degree. C., which makes it possible, in particular, to employ metals such as palladium and its alloys since they diffuse little or not at all in alumina instead of using oxidizable refractory metals such as molybdenum or tungsten which have been the only metals suitable up to the present.
Another aim of the invention is to provide such a substrate in a single firing operation acting on all the layers of the product, thereby eliminating the defects associated with the methods of applying successive firings.
Another aim of the invention is to provide an interconnection substrate having metallized holes for interconnecting one layer to another, a layer to the surface, and one face of the substrate to the other (hereinafter designated "vias") having excellent cylindrical geometrical characteristics, and ensuring a low reject rate after compression and also after sintering, and in which the metallization of the vias, as well as of the rims formed at the outlets of the vias, bonds particularly well to the substrate.
Because of the absence of diffusion, the use of metals such as palladium and its alloys improves the geometrical quality of the conductors. However, it also has a corresponding drawback of the metal bonding poorly to alumina, unlike tungsten or molybdenum, for example, which form intermediate compounds in the diffusion zone at the metal-alumina interface (see, for example, Otsuka et al "Interfacial bond strength in alumina ceramics metallized and cofired with tungsten" in Ceramic Bulletin Vol 60, No. 5, 1981). It will be seen that in one of its aspects, the invention provides a way of escaping from this dilemma.